This invention relates to improved methods and apparatus for igniting and sustaining inductively coupled plasmas produced from radio frequency (RF) power for process operations.
RF plasma is extensively used in a wide variety of applications for carrying out process operations. For example, thermal plasmas can be used to promote chemical reactions because of the high temperatures of the thermal plasma. Alternatively, thermal plasmas are able to promote chemical reactions because of the ability of the energetic electrons to break chemical bonds and allow chemical reactions to occur that would proceed with difficulty under non-plasma conditions.
In other applications, RF power is used to produce non-thermal plasmas, also referred to as non-equilibrium plasmas. The manufacture of semiconductor devices is one area in which non-thermal plasmas are extensively used. During the manufacture of semiconductor devices, etch processes involving RF plasmas and deposition processes involving RF plasmas are used repeatedly during the fabrication process. One of the main benefits of using the non-thermal plasma is the ability of the non-thermal plasma to stimulate chemical reactions that would otherwise require temperatures that are too high for use in the fabrication of semiconductor devices.
RF non-thermal plasmas are also used in cleaning processes in manufacture of semiconductor devices. The non-thermal plasmas are commonly used to strip photoresist materials from semiconductor wafers as part of post etch wafer clean procedures. Resist material is stripped from the surface of the wafers by creating a non-thermal plasma in a gas containing oxidizing species such as oxygen and possibly halogen species that are capable of reacting with and volatilizing the resist material. In some applications, the non-thermal plasma is maintained at a position upstream of the wafer. Reactive species generated in the non-thermal plasma flow downstream and react with the wafer surface for the stripping process. Another cleaning process that uses non-thermal plasmas is the cleaning of reaction chambers used in semiconductor manufacturing.
RF plasmas have also been used for decomposition of chemical compounds that are hazardous or otherwise undesirable. Some of the compounds are highly refractory in nature and are difficult to decompose. Examples of compounds that have been decomposed or abated with plasmas include chlorofluorocarbons (CFC) and perfluorocompounds (PFC).
One frequently encountered problem with standard inductively coupled RF plasma systems is the difficulty of igniting and sustaining the plasma. Plasma ignition is unreliable because coupling an ignition voltage high enough to generate the energetic species needed to produce the plasma is difficult. The voltage required to generate the energetic species is frequently referred to as the breakdown voltage. The breakdown voltage for a gas depends upon a variety of factors. Two major factors are the pressure of the gas and the electronic properties of the gas such as the electronegativity of the gas and its plasma products. The absolute value of the magnitude of the breakdown voltage undergoes a minimum with respect to the pressure of the gas. Specifically, the magnitude of the breakdown voltage increases for plasma ignition at pressures higher or lower than the pressures at which the minimum breakdown voltage occurs. Consequently, igniting plasmas at very low pressures and at high pressures is difficult. The electronegativity of the gas affects the magnitude of the breakdown voltage so that the gas with higher electronegativity requires higher breakdown voltages at every pressure.
Unfortunately for standard inductively coupled plasma technology, the absence of strong electric fields and the absence of strong capacitive coupling make it difficult to overcome the plasma ignition problems resulting from gas pressure and gas electronegativity. At pressures that are too high or too low or for gases with high electronegativities, the required breakdown voltage may equal or exceed the capacity of the RF power source, making plasma ignition unreliable. As a result, it may be necessary to make several attempts to ignite the plasma, greatly reducing the productivity and efficiency of the plasma process. The unreliable plasma ignition can waste valuable process gases, can increase pollution problems, and can ruin valuable product
In addition to the problem of igniting the plasma, there is also the problem of poor plasma stability. After the plasma has been ignited it is possible for the plasma to go out, i.e. become extinguished, because of changes in RF power delivery conditions. For instance, the plasma can go out while performing a process and cause the same unfortunate results that occur for unreliable plasma ignition.
Clearly, there are numerous applications requiring reliable and efficient methods and apparatus for igniting and sustaining inductively coupled RF plasmas. Unfortunately, typical methods and apparatus for old-style inductively coupled RF plasma systems have characteristics that are undesirable for some applications. There is a need for methods and apparatus for igniting and sustaining inductively coupled RF plasmas that are simple to use, operate automatically, and provide high reliability.